This invention relates to electrochemical cell construction, and, in particular, to the construction of electrochemical battery cells.
In the construction of conventional batteries, and, in particular, alkaline batteries, the battery electrodes are typically arranged in spaced parallel relationship, with the positive electrodes interleaved between the negative electrodes to form a succession of positive and negative electrode pairs. In such construction, separators are situated between successive electrodes to maintain the necessary separation therebetween.
Also in this type of battery construction, each of the electrodes is provided with a tab of limited extent or expanse extending from the electrode edge for connecting that electrode to other electrodes of similar polarity. In typical fashion, the tabs of all positive electrodes extend in a common direction and are joined together, either by welding or mechanical means, internal of the battery housing to form an internal positive terminal. All the tabs of the negative electrodes also extend in such common direction and are similarly joined together to form an internal negative terminal. The internal positive and negative terminals, which are located at the end of the battery casing along the common direction, are connected to respective external positive and negative terminals which extend outward of the battery casing, thereby permitting connection of the battery to external loads. Batteries having this general construction, are disclosed, for example, in U.S. Pat. Nos. 3,179,538, 3,640,775 and 4,098,966.
In charge and discharge operation of the aforesaid batteries, current paths are established between the positive and negative electrodes of each electrode pair. These paths extend from the tab of one electrode of a pair, through the body of that one electrode to points on its surface and thence to points on the surface of the other electrode of the pair, through its body and to its tab. Due to the limited expanse of the electrode tabs, and to the tabs extending in a common direction, the established current paths are of different overall length. The resistance of these paths determined by the effective resistance of the electrodes is, therefore, different and is higher for paths of longer length (i.e., paths which include electrode surface points which are farther from the tabs of their respective electrodes).
The aforesaid paths of different resistance established between respective electrodes of the battery electrode pairs support different currents during the charge and discharge operation. As a result, a non-uniform current distribution or density is created over the expanse of each electrode. This non-uniformity is undesirable as it can result in poor utilization of the electrode active material. Furthermore, since the current paths all extend through respective limited expanse regions adjacent the tabs, these regions of the electrodes are used or exercised to a greater degree than the other regions of the electrodes and, hence, are susceptible to increased capacity decay and structure changes. Both these effects (i.e., electrode non-uniform current density and electrode overuse in tab adjacent regions) become more pronounced as electrode size increases (larger batteries), since larger electrodes have greater internal resistance.
To date attempts have been made by battery designers to eliminate these effects. Thus, the use of massive electrodes or profiled electrodes (more massive at the tab region) have been proposed. These approaches are costly, heavy and do not result in a completely uniform current density or distribution across the battery electrodes.
A further effect in conventional batteries of the above type is the creation or generation of heat in the battery during the charge and discharge operation. This heat can damage battery materials and must be controlled. The thermal management problem becomes more extreme in large size batteries, since the perimeter area to total mass decreases. Batteries are generally poor thermal conductors, making it difficult to efficiently remove internally generated heat which can result in large internal temperature gradients. Such gradients contribute to non-uniformity in performance and reduced life.
The aforementioned U.S. Pat. No. 3,179,538 discloses one approach to reducing or controlling battery temperature by causing cooling liquid to flow above the electrode tabs in parallel paths transverse to electrodes.
It is an object of the present invention to provide a battery construction exhibiting a more uniform current density across the battery electrodes.
It is a further object of the present invention to provide such a battery construction further having improved heat control.